1. Field of the Invention
The present invention relates to data processing used in image output equipment such as printers, video printers, and displays, more particularly to a color conversion apparatus and method for use in image output equipment in which image data having a wide gamut of colors are input and a corresponding image is output.
2. Description of the Related Art
An exemplary conventional color conversion apparatus and method are described in Japanese Patent Application Publication No. 2005-184117. The conventional color conversion apparatus includes means for calculating characteristic information about degrees of brightness and saturation of first color data, means for calculating first hue region data values valid in respective hue components by using the first color data, means for using the characteristic information to adjust magnitudes of the first hue region data values to obtain second hue region data values, means for using the difference between the first hue region data values and the second hue region data values to obtain third hue region data values, means for performing a matrix operation using the second and third hue region data values as terms to obtain correction amounts, and color correction means for using the correction amounts to obtain second color data.
This conventional color conversion method enables either the brightness or the saturation of a particular hue component to be modified according to the brightness or saturation of the first color data. If, for example, the image output equipment is a display with a certain gamut of reproducible colors, and the gamut of colors in the image signal formed by the first color data is known, optimal conversion coefficients can be derived from the relation between these two gamuts to obtain a well-displayed image. The gamut of colors of the image signal is defined in terms of a color space, such as the international standard sRGB color space specified in standard 61966-2-1 of the International Electrotechnical Commission (IEC). For broadcast image (video) signals, there is also the BT.709 standard, ‘Parameter values for the HDTV standards for production and international programme exchange’ established by the Radiotelecommunication sector of the International Telecommunication Union (ITU-R). Both the sRGB and BT.709 standards define gamuts based on the characteristics of ordinary cathode ray tube (CRT) displays, and have the same red, green, and blue hues.
Recently, however, various types of displays with wider gamuts of colors than the traditional CRT have been developed and marketed. Furthermore, the gamut defined by the sRGB standard is not necessarily adequate for representing the color distributions of actual objects. For these reasons, color spaces with wider gamuts have been internationally standardized. One example is the xvYCC standard (IEC 61966-2-4), which allows red, green, and blue values greater than 100% or less than 0% (negative values). This standard expands the gamut of the BT.709 standard without changing the red, green, and blue hue values located within the BT.709 gamut.
In the conventional method described above, if images with different gamuts of colors are input as the first image data, to display the images well, the conversion characteristics must be set on the basis of information about the gamut of the first color data. Although such information could conceivably be received with the first color data, in most cases the gamut of the first color data is unknown, making it impossible to produce well-displayed images from all types of first color data.
If scenes are considered individually instead of collectively their gamuts vary greatly, and the very wide xvYCC gamut is unlikely to be fully exploited in most scenes. Since the gamut of colors offered by typical display devices is not wide enough to include the entire xvYCC gamut, images that lie within the gamut of the display tend to be intermixed with images that exceed the gamut of the display, and images that slightly exceed the gamut of the display may be intermixed with images that greatly exceed the gamut of the display. The color conversion characteristics that produce the best displayed image vary for all of these cases.